Thermal shock occurs when dinnerware is heated to a very high temperature quickly. Thermal shock is not considered a defect.
When warming in the microwave, use a plate that best fits the portion size. That way, the foods will heat evenly and will not cause hot spots.
Please also remember to follow your microwave's use and care guidelines for recommended food heating times, and remember to periodically stir food while microwaving. Thermal shock can occur anytime there is a radical temperature change and following these steps should help you to avoid future thermal shock breakage.